1. Field
The present application relates generally to wireless communications, and more specifically to systems, methods, and devices for synchronization of wireless devices in an ad-hoc wireless network.
2. Background
In many telecommunication systems, communications networks are used to exchange messages among several interacting spatially-separated devices. Networks may be classified according to geographic scope, which could be, for example, a metropolitan area, a local area, or a personal area. Such networks would be designated respectively as a wide area network (WAN), metropolitan area network (MAN), local area network (LAN), wireless local area network (WLAN), or personal area network (PAN). Networks also differ according to the switching/routing technique used to interconnect the various network nodes and devices (e.g. circuit switching vs. packet switching), the type of physical media employed for transmission (e.g. wired vs. wireless), and the set of communication protocols used (e.g. Internet protocol suite, SONET (Synchronous Optical Networking), Ethernet, etc.).
Wireless networks are often preferred when the network elements are mobile and thus have dynamic connectivity needs, or if the network architecture is formed in an ad hoc, rather than fixed, topology. Wireless networks employ intangible physical media in an unguided propagation mode using electromagnetic waves in the radio, microwave, infra-red, optical, etc. frequency bands. Wireless networks advantageously facilitate user mobility and rapid field deployment when compared to fixed wired networks.
Devices in a wireless network may transmit and/or receive information to and from each other. To carry out various communications, the devices may need to coordinate according to a protocol. As such, devices may exchange information to coordinate their activities. Improved systems, methods, and devices for coordinating transmitting and sending communications within a wireless network are desired.
FIG. 1a illustrates an example of a prior art wireless communication system 100. The wireless communication system 100 may operate pursuant to a wireless standard, such as an 802.11 standard. The wireless communication system 100 may include an AP 104, which communicates with STAs. In some aspects, the wireless communication system 100 may include more than one AP. Additionally, the STAs may communicate with other STAs. As an example, a first STA 106a may communicate with a second STA 106b. As another example, a first STA 106a may communicate with a third STA 106c although this communication link is not illustrated in FIG. 1a. 
A variety of processes and methods may be used for transmissions in the wireless communication system 100 between the AP 104 and the STAs and between an individual STA, such as the first STA 106a, and another individual STA, such as the second STA 106b. For example, signals may be sent and received in accordance with OFDM/OFDMA techniques. If this is the case, the wireless communication system 100 may be referred to as an OFDM/OFDMA system. Alternatively, signals may be sent and received between the AP 104 and the STAs and between an individual STA, such as the first STA 106a, and another individual STA, such as the second STA 106b, in accordance with CDMA techniques. If this is the case, the wireless communication system 100 may be referred to as a CDMA system.
A communication link that facilitates transmission from the AP 104 to one or more of the STAs may be referred to as a downlink (DL) 108, and a communication link that facilitates transmission from one or more of the STAs to the AP 104 may be referred to as an uplink (UL) 110. Alternatively, a downlink 108 may be referred to as a forward link or a forward channel, and an uplink 110 may be referred to as a reverse link or a reverse channel.
A communication link may be established between STAs. Some possible communication links between STAs are illustrated in FIG. 1a. As an example, a communication link 112 may facilitate transmission from the first STA 106a to the second STA 106b. Another communication link 114 may facilitate transmission from the second STA 106b to the first STA 106a. 
The AP 104 may function as a base station and provide wireless communication coverage in a basic service area (BSA) 102. The AP 104 along with the STAs associated with the AP 104 and that use the AP 104 for communication may be referred to as a basic service set (BSS).
It should be noted that the wireless communication system 100 may not have a central AP 104, but rather may function as a peer-to-peer or ad-hoc network between the STAs. Accordingly, the functions of the AP 104 described herein may alternatively be performed by one or more of the STAs.
FIG. 1b illustrates an example of a prior art wireless communication system 160 that may function as a peer-to-peer or ad-hoc network. For example, the wireless communication system 160 shown in FIG. 1b shows STAs 106a-i that may communicate with each other without the presence of an AP. As such, the STAs, 106a-i may be configured to communicate in different ways to coordinate transmission and reception of messages to prevent interference and accomplish various tasks. In one aspect, the networks shown in FIG. 1b may configured as a “near-me are network” (NAN). In one aspect, a NAN may refer to a network for communication between STAs that are located in close proximity to each other. In some cases the STAs operating within the NAN may belong to different network structures (e.g., STAs in different homes or buildings as part of independent LANs with different external network connections).
In some aspects, a communication protocol used for communication between nodes on the ad-hoc communications network 160 may schedule periods of time during which communication between network nodes may occur. These periods of time when communication occurs between STAs a-i may be known as availability windows. An availability window may include a discovery interval or paging interval as discussed further below.
The protocol may also define other periods of time when no communication between nodes of the network is to occur. In some embodiments, nodes may enter one or more sleep states when the ad-hoc network 160 is not in an availability window. Alternatively, in some embodiments, portions of the stations 106a-i may enter a sleep state when the ad-hoc network is not in an availability window. For example, some stations may include networking hardware that enters a sleep state when the ad-hoc network is not in an availability window, while other hardware included in the STA, for example, a processor, an electronic display, or the like do not enter a sleep state when the ad-hoc network is not in an availability window.
The ad-hoc communication network 160 may assign one node to be a root node. In FIG. 1b, the assigned root node is shown as STA 106e. In ad-hoc network 160, the root node is responsible for periodically transmitting synchronization signals to other nodes in the ad-hoc network. The synchronization signals transmitted by root node 160e may provide a timing reference for other nodes 106a-d and 106f-i to coordinate an availability window during which communication occurs between the nodes. For example, a synchronization message 172a-d may be transmitted by root node 106e and received by nodes 106b-c and 106f-g. The synchronization message 172 may provide a timing source for the STAs 106b-c and 106f-g. The synchronization message 172 may also provide updates to a schedule for future availability windows. The synchronization messages 172 may also function to notify STAs 106b-c and 106f-g that they are still present in the ad-hoc network 160.
One or more of the nodes in the ad-hoc communication network 160 may function as branch synchronization nodes. A branch synchronization node may retransmit both availability window schedule and master clock information received from a root node. In some embodiments, synchronization messages transmitted by a root node may include availability window schedule and master clock information. In these embodiments, the synchronization messages may be retransmitted by the branch synchronization nodes. In FIG. 1b, STAs 106b-c and 106f-g are shown functioning as branch-synchronization nodes in the ad-hoc communication network 160. STAs 106b-c and 106f-g receive the synchronization message 172a-d from root node 106e and retransmit the synchronization message as retransmitted synchronization messages 174a-d. By retransmitting the synchronization message 172 from root node 106e, the branch synchronization nodes 106b-c and 106f-g may extend the range and improve the robustness of the ad-hoc network 160.
The retransmitted synchronization messages 174a-d are received by nodes 106a, 106d, 106h, and 106i. These nodes may be characterized as “leaf” nodes, in that they do not retransmit the synchronization message they receive from either the root node 106e or the branch synchronization nodes 106b-c or 106f-g. 
FIG. 1c illustrates an example of a prior art wireless communication system implementing an ad-hoc network 160. In FIG. 1c, the ad-hoc network 160 is shown after the root node 106e of FIG. 1c has left the ad-hoc network 160. Root node 106e may leave the network 160 for one of several reasons. For example, a hardware failure on root node 106e may prevent it from continuing to transmit synchronization messages 172 as shown in FIG. 1c. Alternatively, root node 106e may be a mobile device, and as such the root node 106e may move out of range of the ad-hoc network 160. As a result of the root node 106e being out of range, any synchronization messages transmitted by root node 106e will not be received by other nodes of the ad-hoc network 160.
In response to root node 106e leaving the ad-hoc network 160, at least some of the devices included in the ad-hoc network 160 may lose synchronization with other nodes of the ad-hoc network. For example, because the root node 106e may no longer be transmitting synchronization messages, an availability window between the remaining nodes of the ad-hoc network may not be maintained. In response, the remaining nodes of the network may renegotiate to determine a new root. For example, as shown in FIG. 1c, the renegotiation process may include, as an initial step, each node of the ad-hoc network 160 designating itself as a new root node. As such, each node may begin transmitting synchronization messages, as shown by synchronization messages 180a-h. Since the ad-hoc network 160 may not support multiple root nodes, additional negotiation may be performed between the remaining nodes to determine which of the remaining nodes is a new root node. For example, in some implementations, the signal strength of the synchronization messages 180a-h as received by each of the remaining nodes may be used to determinate the new root.
This resynchronization process may consume processing resources of each of the devices in the ad-hoc network 160. Additionally, a minimum amount of elapsed time may be necessary for the devices of the ad-hoc network 160 to resynchronize. During this resynchronization time period, communication between nodes of the ad-hoc network 160 may be disrupted.